Careful of the towel idea as they tend to breed bacteria. Lots of people using glass tanks foil the lid or plexiglass the lid to help with the humidity.

If humidity is an issue still, many use sphagnum moss with excellent results

Rheostats work well and require little daily tinkering if the external environment stays pretty darn constant. If your room environment changes temps throughout the day and night, a thermostat will work much better.